Just give her some deep looks right into the eyes and see how she responds. If she doesn't even look you in eyes that's a pretty clear indication that she's oblivious. After making eye contact? Well, that varies from person to person, you just need to pick up on the subtleties.

-Deffinitely flirting (walking close to you, looking over at you for no apparent reason, being really nice, giving you or asking for alot of hugs, talking to you more then she talks to her other guy friends)

Asking her out on a date:-Just straight up ask her "Would you like to go to *insert place,like movies* with me?"

"Do you want to go to the movies with me?"
vs
"Hey, I'm going to the movies on Friday, your welcome to come."

The first is much easier to say no to because it creates the image of "need" you need her to come with you. She's enabling you to go. Creates a needier image of yourself which is not confident and girls pick up this.

The second is not a question. Your going ragardless, your just saying you woulnd't mind if she comes along. It creates a enigmatic persona which makes you appear interesting and confident.

Women like confidence and humour. Use them and you will do well with women.

As for how you know if she likes you, tell her jokes, make her laugh. Look at her smile, look at her lips. Touch her hair, touch her arm, touch her a shoulder. Tease her with touch if she likes it, go to hold her hand and pull away.

If she does not object it's a good sign.
If she laughs alot around you another good sign.

Last but not least the kiss test! If you can touch her hair and I mean like able to stroke it without her getting uncomfortable, she will let you kiss her.

ok....what do you do when the girl says "not just yet" like does that really mean not just yet or is that no?

Honestly in my experience it's best to take people at their word instead of wandering instead of worrying about endless possibilities of unspoken meanings behind what they said, ESPECIALLY in the dating world. If she said "just not yet" it means she hasn't made up her mind, isn't ready, or isn't looking for that kind of relationship right now and doesn't want to hurt your feelings. (If she meant some kind of implied hidden meaning you were supposed to pick up on, because some people do like to play games like that, then it's her fault for not conveying it and it would mean she isn't worth your time because she cannot respect your honest feelings)

The correct response would be to back off on the romantic advances a bit. Give her some room, and get to know each other as friends. The good news is you have offered the invitation to her. The ball is in her court; she will now come to you when she is ready.